It is known that plant viruses represented by tobamovirus, after infecting a plant, proliferate in the plant and rapidly spread systemically throughout the plant while producing viral coat protein in large amounts.
So far, some gene engineering systems for these plant viruses have been constructed and some attempts have been carried out to introduce a foreign gene into a plant using said systems. For example, there is a process wherein a coat protein gene is replaced with a foreign gene (Takamatsu et al, EMBO J., 6:307-311 (1987)), and a process wherein a coat protein gene and a foreign gene are directly joined so as to produce a fused protein (Takamatsu et al, FEBS Lett., 269:73-76 (1990 )).
However, all of the plant viruses used in these known processes have a drawback in that they do not spread systemically in a plant (i.e., they do not have systemic infectivity), and thus, it is impossible to introduce a useful property and to produce a useful protein throughout a whole plant.
For a plant virus to exhibit systemic infectivity, particle formation with wild type coat protein is essential (Saito et al, Virology, 176:329 (1990)). As to existing plant virus vectors, since the coat protein is not produced (replacement-type vector), or the coat protein is in the form of a fused protein, resulting in a big change in properties (direct-joining type vector); then particles cannot be formed and systemic infectivity is not exhibited.
In addition, where existing plant virus vectors are used to express a foreign gene, it is very difficult to isolate and purify the foreign gene product from the plant into which the foreign gene was introduced.
The reason is that to establish (1) simple apparatus and economy, (2) high recovery, (3) high purity and (4) good reproduction, which are the goals of isolation and purification, a combination of operations, such as differential precipitation, desalting, concentration, and various chromatographies is essential; however, a series of these operations usually takes one-half to one month to complete, and they are often time- and labor-consuming. Even assuming these operations are simple and rapid, a plant includes proteins whose properties, such as molecular weight, isoelectric point, affinity to a solvent, are similar to those of the foreign gene product; and therefore it is difficult to prevent loss of the foreign gene product during the isolation and purification process. Thus, high recovery of the foreign gene product is not expected.